Relativistic simultaneity and causality

Relativistic simultaneity and causality V. J. Bolós 1,, V. Liern 2, J. Olivert 3 1 Dpto. Matemática Aplicada, Facultad de Matemáticas, Universidad de Valencia. C/ Dr. Moliner 50. 46100, Burjassot Valencia), Spain. e-mail: vicente.bolos@uv.es 2 Dpto. Economía Financiera y Matemáticas, Facultad de Economía, Universidad de Valencia. Avda. Tarongers s/n. 46022, Valencia, Spain. e-mail: vicente.liern@uv.es 3 Dpto. Astronomía y Astrofísica, Facultad de Física, Universidad de Valencia. C/ Dr. Moliner 50. 46100, Burjassot Valencia), Spain. e-mail: joaquin.olivert@uv.es June 2001 Revised: March 2005) Abstract We analyze two types of relativistic simultaneity associated to an observer: the spacelike simultaneity, given by Landau submanifolds, and the lightlike simultaneity also known as observed simultaneity), given by past-pointing horismos submanifolds. We study some geometrical conditions to ensure that Landau submanifolds are spacelike and we prove that horismos submanifolds are always lightlike. Finally, we establish some conditions to guarantee the existence of foliations in the spacetime whose leaves are these submanifolds of simultaneity generated by an observer. 1 Introduction It is well known that some problems related with simultaneity have not been solved yet. A great number of works treat the local character of relativistic simultaneity accepting that Landau submanifolds [1] generated by an observer are leaves of a spacelike foliation. However, the fulfillment of this property cannot be ensured on any neighborhood without assuming some additional geometrical conditions. Therefore, when working on a neighborhood where this property does not hold, some difficulties in setting a successful dynamical study arise. The study of some of these conditions is the main objective of this paper. In this work, we consider two types of simultaneities: spacelike simultaneity, which describes those events that are simultaneous in the local inertial proper system of the observer; and lightlike or observed) simultaneity, which describes those events which the observer observes as simultaneous although they are not simultaneous in its local inertial proper system. The sets of spacelike simultaneous events and lightlike simultaneous events determine the Landau submanifolds and the past-pointing horismos [2] submanifolds respectively. Work partially supported by a grant of the Ministerio de Educación y Cultura, Spain. 1

Our next concern is the causality related to these types of simultaneity, because we should be able to guarantee, for instance, that Landau submanifolds are spacelike in a given neighborhood. For this, we introduce a new concept, the tangential causality, more general than causality, and we prove that every Landau submanifold L p,u is p-tangentially spacelike, but it is not necessarily spacelike. On the other hand, we prove that every horismos submanifold E p is p-tangentially lightlike and lightlike. In physics, it is usual to work with synchronizable timelike vector fields. In most cases, the leaves of the orthogonal foliation are considered simultaneity submanifolds. In this work it is proved that given an observer β i.e. a C timelike curve), there exists, on a small enough tubular neighborhood of this observer, a synchronizable timelike vector field containing the 4-velocity of β. Moreover, this vector field is orthogonal to a Landau submanifolds foliation. But we can not assure the existence of these kind of vector fields on any convex normal neighborhood. On the other hand, it is also proved that given an observer, there exists a foliation whose leaves are past-pointing horismos submanifolds of events of the observer, and it is well defined on any convex normal neighborhood. 2 Preliminary concepts In what follows, M, g) will be a 4-dimensional lorentzian spacetime manifold. Given v a vector, v will denote the subspace orthogonal to v. Let p M. An open neighborhood N 0 of the origin in T p M is said to be normal if the following conditions hold: i) The mapping exp p : N 0 N p is a diffeomorphism, where N p is an open neighborhood of p. ii) Given X N 0 and t [0, 1] we have that tx N 0. For a given event p M, an open neighborhood N p of p is a normal neighborhood of p if N p = exp p N 0, where N 0 is a normal neighborhood of the origin in T p M. Finally, an open set V in M, which is a normal neighborhood of each one of its points, is a convex normal neighborhood. These neighborhoods are useful to obtain a dynamical study of simultaneity. Moreover, the Whitehead Lemma asserts that given p M and a neighborhood U of p, there always exists a convex normal neighborhood V of p such that V U [3]. Since we are not going to make a global study, we will consider the spacetime M as a convex normal neighborhood in order to simplify. So, given two events in M, there exists a unique geodesic containing them. We are going to introduce two static ways to analyze simultaneity: Landau and pastpointing horismos submanifolds. Given u T p M the 4-velocity of an observer at p, and the metric tensor field g, we consider the submersion Φ : M R given by Φ q) := g exp p q, u ). The fiber L p,u := Φ 0) 1) is a regular 3-dimensional submanifold called Landau submanifold of p, u). In other words, see Figure 1). The next result is given in [1]: L p,u = exp p u Theorem 2.1. Given u T p M the 4-velocity of an observer at p, there exists a unique regular 3-dimensional submanifold L p,u such that T p L p,u = u and whose points are simultaneous with p in the local inertial proper system of u. 2

Figure 1: Construction of the Landau submanifold L p,u by means of the exponential map. Figure 2: Construction of the horismos submanifolds E p and E + p by means of the exponential map. On the other hand, defining the submersion ϕ : M {p} R given by ϕ q) := g expp q, exp p q ), the fiber E p := ϕ 0) 2) is a regular 3-dimensional submanifold, called horismos submanifold of p, which has two connected components [3]. We will call past-pointing respectively future-pointing) horismos submanifold of p, Ep resp. E p + ), to the connected component of 2) in which, for each event q M {p}, the preimage exp p q is a past-pointing respectively future-pointing) lightlike vector. In other words, Ep = exp p Cp ; E p + = exp p C p +, where Cp and C p + are the past-pointing and the future-pointing light cones of T p M respectively see Figure 2). The events in a Landau submanifold L p,u are simultaneous with p in the local inertial proper system of p i.e. they are synchronous with p), but they are not observed as simultaneous in the sense that they are not observed at the same instant) by any observer. On the other hand, the events in a past-pointing horismos submanifold Ep are observed as simultaneous by any observer at p, since they belong to light rays which arrive at p. So, a Landau submanifold L p,u defines an intrinsic simultaneity for an observer with 4-velocity u at p, and a past-pointing horismos submanifold Ep defines an observed simultaneity for any observer at p. In general, we will call both, Landau and past-pointing horismos submanifolds, simultaneity submanifolds. 3

3 Tangential causality Our aim is to study the causality of simultaneity submanifolds, specially to identify those cases in which we can assure that a Landau submanifold L p,u is spacelike and a past-pointing horismos submanifold Ep is lightlike. But first, we are going to introduce a new concept called tangential causality, that generalizes the classic concept of causality and let us assure that the p-tangential causality of L p,u and Ep is always spacelike and lightlike respectively see Proposition 3.1 below). Let V be a 4-dimensional vector space regarded as a C manifold. It can be canonically identified with any of its tangent spaces: for each v V there exists a unique isomorphism φ v : T v V V such that ω φ v w) = w ω) 3) for all w T v V and for all ω V. If, moreover, V, g) is a lorentzian vector space, we define a 0, 2)-tensor field g on V by g w, w ) := g φ v w, φ v w ) 4) where v V and w, w T v V. Then, V, g) is a lorentzian manifold [3]. Let p be in M, we can apply this to T p M because T p M, g p ) is a lorentzian vector space, where g p := g TpM. So, for each v T p M we can define a canonical isomorphism φ v of T v T p M) onto T p M satisfying 3). If we define g p on T v T p M) from g p according to 4)), then T p M, g p ) is a lorentzian manifold. Definition 3.1. Let N be a regular submanifold of M and p N. The p-tangential submanifold of N is given by exp p N, 5) considered as a regular submanifold in T p M. Given q N, we define the p-tangential causality of N at q as the causality of exp p N at exp p q, using g p. If this causality is the same at every point of exp p N, then we define the p-tangential causality of N as the causality of exp p N at any point. The p-tangential causality can be interpreted as an observed causality at p, because an observer detects the events of the spacetime through its tangent space. It is easy to prove that T q N = exp p v Tv exp p N )), 6) where N is a regular submanifold of M, p, q N, and v = exp p q. As a particular case, since exp p 0 = φ 0 [2], we have T p N = φ 0 T0 exp p N )). Then, the causality of N at p coincides with the p-tangential causality of N at 0. However, given v exp p N such that v 0, the causality of N at p is not necessarily the same as the p-tangential causality of N at v. Applying the tangential causality concept to simultaneity submanifolds, we obtain the next result: Proposition 3.1. Given p M, and u T p M the 4-velocity of an observer at p, we have that a) the p-tangential causality of L p,u is spacelike. b) the p-tangential causality of E p is lightlike. 4

Proof. a) Given v exp p L p,u such that v 0, we have that g u, v) = 0. We define g u : T p M R v g u v ) := g u, v ). ) So, exp p L p,u = gu 0). Hence, given w T v T p M), we have that w T v exp p L p,u if and only if w g u ) = 0, if and only if g u φ v w) = 0 by 3)), if and only if g u, φ v w) = 0 by 7)), if and only if g p φ v u, w ) = 0 by 4)). Then 7) ) T v exp p L p,u = φ v u ). 8) Moreover, φ v u is timelike because g p φ v u, φ v u ) = g u, u) =. Thus, 8) is a spacelike subspace and hence exp p L p,u is a spacelike submanifold of T p M, g p ). b) Analogously, ) T v exp p E p = φ v v ). 9) Since φ v v is lightlike, we have that exp p E p is a lightlike submanifold of T p M, g p ). 4 Causality of simultaneity submanifolds Definition 4.1. Given p M and v T p M, we define v q := τ pq v, 10) where q M and τ pq is the parallel transport along the unique geodesic segment from p to q note that we are considering M as a convex normal neighborhood, and so there exists a unique geodesic containing p and q). It is clear that 10) depends differentiably on q and thus v is a vector field, named vector field adapted to v. The vector field v adapted to v has the same causal character as v, since parallel transport keeps orthogonality. Definition 4.2. Let U be a timelike vector field on an open neighborhood U. U is synchronizable on U if and only if its orthogonal 3-distribution U is a foliation on U. Then, U is called the physical spaces 3-foliation of U and it is also denoted by S U. Theorem 4.1. Given p M, and u T p M the 4-velocity of an observer at p, we have that a) if u is synchronizable on an open neighborhood of q L p,u, then L p,u is spacelike at q. b) E p is always lightlike. Proof. a) Let us call v := exp p q. By 6) and 8) we have )) T q L p,u = exp p v Tv exp φ p L p,u = expp v v u ) ). 11) Let w be in φ v u ). Then g u, φv w) = 0, and hence g u, φ v w) ) = 0 because parallel transport keeps orthogonality. So, φ v w) q and v q are in u q). Since u is synchronizable on an open neighborhood of q, u ) is a foliation in this open neighborhood that is, it is involutive) and hence [ v, φ v w) ] q u q). Let us denote θ X) Y ) the Lie bracket [X, Y ] where X, Y are vector fields. Then θ v ) φ v w) ) q u q ). Using induction over n N θ v ) n φ v w) ) q u q). 12) 5

Since exp p v w = n=0 ) n n + 1)! θ v ) n φ v w) ) q 13) see [4]), applying 12) in 13), we have exp p v w u q). So, by 11), we have T q L p,u = u q), 14) because they have the same dimension. Concluding, L p,u is spacelike since u q is timelike. b) Given q E p and v := exp p q, by 6) and 9) we have )) T q E p = exp p v Tv exp φ p E p = expp v v v ) ). 15) Let w be in φ v v ). Then g v, φv w) = 0, and hence g v, φ v w) ) = 0 because parallel transport keeps orthogonality. So, φ v w) q and v q are in v q ). Since torsion vanishes, [ v, φ v w) ] q = v φ v w) ) q φvw) v ) q, 16) but v φ v w) ) q = 0 17) because φ v w) is parallelly transported along the geodesic that joins p and q i.e., the integral curve of v passing through q). Moreover, given X, Y, Z three vector fields, we have Zg X, Y ) = g Z X, Y ) + g Z Y, X) see [4]). Taking X, Y = v and Z = φ v w), we obtain g φvw) v, v ) = 0. 18) Applying 17) and 18) in 16), and using the previous notation for the Lie bracket, we have θ v ) φ v w) ) q u q). Using induction over n N θ v ) n φ v w) ) q v q ). 19) Applying 19) in 13), we have exp p v w v q ). So, by 15), we have T q E p = v q ), 20) because they have the same dimension. Concluding, E p is lightlike since v q is lightlike. Under the hypotheses of Theorem 4.1, by 14) and 20), we have T q L p,u = τ pq u ; T q E p = τ pq v, since u q) = τpq u and v q ) = τpq v. It is important to remark that if the adapted vector field u is not synchronizable in any open neighborhood of q, then we can not assure that L p,u is spacelike at q, but we can always assure that the p-tangential causality of L p,u at q is spacelike, by Proposition 3.1. Remark 4.1. Since L p,u is spacelike at p, we can always assure that there exists a small enough open neighborhood of p in which L p,u is spacelike. 6

Figure 3: Landau foliation L β. 5 Simultaneity foliations Since we are supposing that the spacetime M is a convex normal neighborhood, we can define Landau and horismos submanifolds at any event p. In particular, given β : I M an observer where I is an open real interval and β is parameterized by its proper time), we can define the sets of Landau and horismos submanifolds { Lβt) } ; { E βt) } ; { E + βt) where L βt) denotes L βt), βt). Our aim in this section is to study these sets of Landau and horismos submanifolds as leaves of a foliation. Theorem 5.1. Let β : I M be an observer and p βi. There exists a convex normal neighborhood V of p where a foliation L β called Landau foliation generated by β) is defined, and whose leaves are { L βt) V } see Figure 3). Proof. Let us introduce some definitions that can be found in [5]. Let N be a submanifold of M, we define the normal tangent fiber bundle T N as the fiber bundle composed by the subspaces T p N. On an open neighborhood of the zero section O { T N ) = 0p T p N : p N } we can define the normal exponential map of N as follows: } exp : T N M v T p N exp p v. Considering βi as a submanifold of M, we can define the normal exponential map of βi on an open neighborhood of the zero section of T βi. Using an analogous reasoning given in [5], it is proved that for all p βi there exists an small enough open neighborhood V of p that we can assume to be convex normal neighborhood) in which the normal exponential map of βi is a diffeomorphism. So, given q V, it belongs to one and only one Landau submanifold of the family { L βt) V }. Since these submanifolds are regular, they are the leaves of a foliation on V. 7

Figure 4: Intersection of two Landau submanifolds L p,u and L p,u in Minkowski spacetime, where p, p are events of a not geodesic observer β and u, u are the 4-velocities of this observer at p, p respectively. According to Theorem 5.1 and Remark 4.1, there exists a small enough tubular neighborhood of βi where the Landau foliation generated by β is well defined and spacelike. So, there exists a synchronizable future-pointing timelike unit vector field defined on this tubular neighborhood) orthogonal to the Landau foliation L β. The integral curves of this vector field are a congruence of observers orthogonal to L β. But, given a convex normal neighborhood, we can not assure that the Landau foliation generated by β is well defined on it, because the leaves can intersect themselves. In fact, it is usual that L βt1) L βt2) for t 1, t 2 I, t 1 t 2 ; for instance, in Minkowski spacetime if β is not geodesic see Figure 4). Unlike the Landau foliations, the horismos foliations past-pointing and future-pointing) are well defined on any convex normal neighborhood, because their leaves do not intersect in any case: Theorem 5.2. Let β : I M be an observer. There exists a foliation E β called pastpointing horismos foliation generated by β) defined on } E βt) {E and whose leaves are βt) see Figure 5). Analogous for future-pointing horismos. Proof. It is proved in [3] that, in a convex normal neighborhood, past-pointing or futurepointing) horismos submanifolds of different events of a given observer do not intersect. Since they are regular submanifolds, it is clear that they form a foliation. According to Theorems 4.1b) and 5.2, the past-pointing and future-pointing) horismos foliation generated by an observer is always well defined on any convex normal neighborhood) and lightlike. 6 Discussion and open problems If we work in the framework of spacelike simultaneity i.e., simultaneity in the local inertial proper system of the observer, see [1]) there appear several serious mathematical problems related with Landau submanifolds: it can not be assured that they were spacelike at any point see Theorem 4.1) and the leaves of a Landau foliation associated to a given observer can intersect themselves, even working in a convex normal neighborhood see Figure 4). On the other hand, all these problems disappear if we work in the framework of lightlike or 8

Figure 5: Past-pointing horismos foliation E β. observed) simultaneity, i.e. working with past-pointing horismos submanifolds instead of Landau submanifolds. Moreover, given an observer at an event p with 4-velocity u, the events of its Landau submanifold L p,u do not affect the observer at p in any way, since both electromagnetic and gravitational waves travel at the speed of light. On the other hand, the events of its pastpointing horismos submanifold Ep are precisely the events that affect and are observed by the observer at p, i.e. the events that exist for the observer at p. So, we can say what you see is what happens. More arguments in favor of lightlike simultaneity against spacelike simultaneity are discussed in [6]. To conclude, we are going to present the main open problems, that are related with Landau submanifolds: A Landau submanifold L p,u is spacelike at any point of any convex normal neighborhood of p. There is not any satisfactory proof of this fact, neither a counterexample showing that it is false. Given β : I M an observer, we have that L βt1) L βt2) = for any t 1, t 2 I, t 1 t 2, if and only if...? Supposing that M is a convex normal neighborhood.) In Minkowski spacetime the answer is if and only if β is geodesic, but this property has not been yet generalized to general relativity. It would be useful to characterize when an observer has focal points see [5]). References [1] J. Olivert. On the local simultaneity in general relativity. J. Math. Phys. 21 1980), 1783 1785. [2] J. K. Beem, P. E. Ehrlich. Global Lorentzian Geometry. Marcel Dekker, New York 1981). 9

[3] R. K. Sachs, H. Wu. Relativity for Mathematicians. Springer Verlag: Berlin, Heidelberg, New York 1977). [4] S. Helgason. Differential Geometry and Symmetric Spaces. Academic Press, London 1962). [5] T. Sakai. Riemannian Geometry. American Mathematical Society, New York 1996). [6] V. J. Bolós. Lightlike simultaneity, comoving observers and distances in general relativity. J. Geom. Phys. 56 2006), 813 829 arxiv:gr-qc/0501085). 10